#190 Allow easy use of TLS with pop3

[Michael Schlenker]

As more and more services allow to use POP3 with
SSL/TLS it would be a nice move to support SSL within
the POP3 package.

For SSL enabled servers listening only on an SSL port
this is trivial, just use the tls::socket instead of
the normal one for the connect.

Using the POP3 STLS Command (StartTLS), is a bit more
complex, but not that hard to do, its just an extra
step in authentification and some logic to do a
tls::import on the open channel.

Michael